Mumble is an open source VOIP client that is available for windows and linux. However, this VOIP is a client-server one , hence requires both the clients to connect to a server. While you may choose a public server in the list when launching the desktop application of mumble, it's better to configure and setup your own mumble server so that you have more choice of freedom and you can even set the voice quality and codec that can use lesser bandwidth. Further, mumble is more like Push to Talk, so it uses very little bandwidth (I was torrenting on my 256k connection while using mumble with just 2 people in the room and still it was fine!)